1. Field of the Invention
The present invention relates to a probe for an electrooptic sampling oscilloscope that couples an electrical field generated by a measured signal and an electrooptic crystal, inputs a beam into this electrooptic crystal, and measures the waveform of the measured light signal by the state of the polarization of the input light.
This application is based on Japanese Patent Application, No. Hei 10-294567 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
It is possible to couple an electrical field generated by a measured signal with an electrooptic crystal, input a laser beam into this electrooptic crystal, and observe the waveform of the measured signal by the state of the polarization of the laser beam. It is possible pulse the laser beam, and observe with an extremely high time resolution when sampling the measured signal. The electrooptic sampling oscilloscope uses an electrooptic probe exploiting this phenomenon.
When this electrooptic sampling oscilloscope (hereinbelow, referred to as an xe2x80x9cEOS oscilloscopexe2x80x9d) is compared to a conventional sampling oscilloscope using an electrical probe, the following characteristics have received much attention:
1. It is easy to observe the signal because a ground wire is unnecessary.
2. Because the metal pin at the end of the electrooptic probe is not connected to the circuit system, it is possible to realize high input impedance, and as a result of this, there is almost no degradation of the state of the measured point.
3. By using an optical pulse, broadband measurement up to the GHz order is possible.
The structure of a probe for an EOS oscilloscope in the conventional technology will be explained using FIG. 7. In the electrooptic probe 1 shown in FIG. 7, a probe head 3 comprising an insulator is mounted on the end terminal of the metallic probe body 2, and a metallic pin 3a is fit into the center. Reference numeral 4 is an electrooptic element, a reflecting film 4a is provided on the end surface on the metallic pin 3a side, and is in contact with the metallic pin 3a. Reference numeral 5 is a xc2xd wavelength plate, and reference numeral 6 is a xc2xc wavelength plate. Reference numeral 7 and 8 ate polarized beam splitters. Reference numeral 9 is a xc2xd wavelength plate, and reference numeral 10 is a Faraday element. Reference numeral 12 is a collimator lens, and reference numeral 13 is a laser diode. Reference numerals 14 and 15 are condensing lenses, and reference numerals 16 and 17 are photodiodes.
In addition, the two polarized beam splitters 7 and 8, the xc2xd wavelength plate 9, and the Faraday element 10 comprise an isolator 19 that transmits the light emitted by the laser diode 13, in order to split the light reflected by the reflecting film 4a. 
Next, referring to FIG. 7, the optical path of the laser beam emitted from the laser diode 13 is explained. In FIG. 7, reference letter xe2x80x98Axe2x80x99 denotes the optical path of the laser beam.
First, the laser beam emitted from the laser diode 13 is converted by the collimator lens 12 into a parallel beam that travels straight through the polarized beam splitter 8, the Faraday element 10, the xc2xd wavelength plate 9, and the polarized light beam splitter 7, and then transits the xc2xc wavelength plate 6 and the xc2xd wavelength plate 5, and is incident on the electrooptic element 4. The incident light is reflected by the reflecting film 4a formed on the end surface of the electrooptic element 4 on the side facing the metallic pin 3a. 
The reflected laser beam transits the xc2xd wavelength plate 5 and the xc2xc wavelength plate 6, one part of the laser beam is reflected by the polarized light beam splitter 7, condensed by the condensing lens 14, and incident on the photodiode 16. The laser beam that has transited the polarized light beam splitter 7 is reflected by the polarized beam splitter 8, condensed by the condensing lens 15, and incident on the photodiode 17.
Moreover, the angle of rotation of the xc2xd wavelength plate 5 and the xc2xc wavelength plate 6 is adjusted so that the strength of the laser beam incident on the photodiode 16 and the photodiode 17 is uniform.
Next, using the electrooptic probe 1 shown in FIG. 7, the procedure for measuring the measured signal is explained. When the metallic pin 3a is placed in contact with the measurement point, due to the voltage applied to the metallic pin 3a, at the electrooptic element 4 this electrical field is propagated to the electrooptic element 4, and the phenomenon of the altering of the refractive index due to the Pockels effect occurs. Thereby, the laser beam emitted from the laser diode 13 is incident on the electrooptic element 4, and when the laser beam is propagated along the electrooptic element 4, the polarization state of the beam changes. Additionally, the laser beam having this changed polarization state is reflected by the reflecting film 4a, condensed and incident on the photodiode 16 and the photodiode 17, and converted into an electrical signal.
Along with the change in the voltage at the measurement point, the change in the state of polarization by the electrooptic element 4 becomes the output difference between the photodiode 16 and the photodiode 17, and by detecting this output difference, it is possible to observe the electrical signal applied to the metallic pin 3a. 
Moreover, in the above-described electrooptic probe 1, the electrical signals obtained from the photodiodes 16 and 17 are input into an electrooptic sampling oscilloscope, and processed, but instead, it is possible to connect a conventional measuring device such as a real time oscilloscope at the photodiodes 16 and 17 via a dedicated controller. Thereby, it is possible to carry out simply broadband measurement by using the electrooptic probe 1.
In the manner described above, in the signal measurement using the electrooptic probe 1, because a metallic pin 3a must contact the measurement point, in this case, a shock is applied to the metallic pin 3a, and as a result, there is the concern that damage may occur to the electrooptic element 4.
In addition, the electrooptic probe 1 described above has a structure wherein a laser beam is incident on the reflecting film 4a, with which the metallic pin 3a is in contact, and is then reflected, and thus when the position of the metallic pin 3a is moved, the position of the reflecting film 4a, etc., shifts, and there is the problem that its function as an optical system is lost.
In consideration of the above described problems, it is an object of the present invention to solve this problem by improving the shock resistance of the electrooptic probe by anchoring the position of the metallic pin with respect to the probe head.
In order to solve the above problem, the following means are used.
A first aspect of the present invention is an electrooptic probe wherein:
a light path between a base terminal and an end terminal of the probe body is formed within the probe body;
at the end of the light path on the base terminal side of the probe body, a laser diode is disposed;
at the other end of the light path on the end terminal side of the probe body, an electrooptic element is disposed;
at the end surface of the electrooptic element on the end terminal side of the probe body, a reflecting film is formed;
the laser beam emitted from the laser diode is incident on the electrooptic element via the optical path, this incident beam is reflected by the reflecting film, and furthermore, this reflected light is separated and converted into an electric signal; and wherein
the electrooptic element is supported at least from the end terminal side of the probe body by a probe head member that serves as the end terminal of the probe body;
an insertion hole from the outside to the reflecting film is formed on the probe head member;
the metallic pin is inserted in the insertion hole so that one end contacts the reflecting film and the other end projects from the probe head; and
said insertion hole is formed so that the radial dimension of its outer side is large in comparison to the radial dimension of its reflection film side.
Because of this kind of construction, in this electrooptic probe, the one end of the metallic pin is formed so as to conform to the shape of the insertion hole, and thereby it is possible to prevent damage from occurring to the electrooptic element due to the metallic pin being inserted into the probe head member more than necessary.
A second aspect of the present invention is an electrooptic probe according to the first aspect wherein the photodiode and the laser diode are connected to an electrooptic sampling oscilloscope; and
the laser diode generates a laser beam as a pulsed beam based on the control signal from the electrooptic oscilloscope.
A third aspect of the present invention is an electrooptic probe according to the second aspect wherein the insertion hole is formed having a tapered shape that gradually narrows from outside towards the reflection film side.
A fourth aspect of the present invention is an electrooptic probe according to the third aspect wherein the one end of the metallic pin is formed as a radially changing part such that the radial dimension becomes smaller from the one other end side to the one end side.
A fifth aspect of the present invention is an electrooptic probe according to the third aspect characterized in having a slit that passes through the metallic pin in the direction of the diameter being provided at the one end of the metallic pin.
Because of being structured in this manner, the one end of the metallic pin can be deformed so as to conform to the shape of the insertion hole provided in the probe head.
A sixth aspect of the present invention is an electric probe according to the second aspect characterized in the insertion hole being formed provided with steps on the inner surface, whose radial dimension becomes smaller from the outer side towards the reflecting film side.
A seventh aspect of the present invention an electrooptic probe according to the sixth aspect wherein the end of the metallic pin is formed as a radially changing part such that the radial dimension becomes smaller from the one end towards the other end.
A eighth aspect of the present invention is an electrooptic probe according to the sixth aspect characterized in having a slit that passes through the metallic pin in the direction of the diameter being provided at the one end of the metallic pin.
Because of being structured in this manner, in the electrooptic probe according to the eighth aspect, the one end of the metallic pin can be deformed so as to conform to the shape of the insertion hole provided on the probe head.
A ninth aspect of the present invention is an electrooptic probe according to the first aspect characterized in a laser diode generating a continuous laser beam.
In this manner, a continuous beam is generated from the laser diode, and thereby it is possible to obtain a continuous output from the photodiode, and therefor it is possible to make measurements by connecting a photodiode to a conventional general use measuring device such as a real time oscilloscope.
A tenth aspect of the present invention is an electrooptic probe according to the ninth aspect wherein the insertion hole is formed having a tapered shape that gradually narrows from the outside towards the reflection film side.
A eleventh aspect of the present invention is an electrooptic probe according to the tenth aspect wherein the one end of the metallic pin is formed as a radially changing part such that the radial dimension becomes smaller from the one end towards the other end.
A twelfth aspect of the present invention is an electrooptic probe according to the tenth aspect characterized in having a slit that passes through the metallic pin in the direction of the diameter being provided at the one end of the metallic pin.
Because of being structured in this manner, in the electrooptic probe according to the twelfth aspect, the one end of the metallic pin can be deformed so as to conform to the shape of the insertion hole provided on the probe head.
A thirteenth aspect of the present invention is an electric probe according to the ninth aspect characterized in the insertion hole being formed provided with steps on the inner surface whose radial dimension becomes smaller from the outside towards the reflecting film side.
A fourteenth aspect of the present invention is an electrooptic probe according to the thirteenth aspect wherein the end of the metallic pin is formed as a radially changing part such that the radial dimension becomes smaller from the one end towards the other end.
A fifteenth aspect of the present invention is an electrooptic probe according to the thirteenth aspect characterized in having a slit that passes through the metallic pin in the direction of the diameter being provided at the one end of the metallic pin.
Because of being structured in this manner, in the electrooptic probe according to the fifteenth aspect, the one end of the metallic pin can be deformed so as to conform to the shape of the insertion hole provided on the probe head.
A sixteenth aspect of the present invention is an electrooptic probe according to the first aspect characterized in the dimension of the one end of the insertion hole being larger than the spot size of the beam incident to the electrooptic element.
Because of being structured in this manner, in the electrooptic probe according to the sixteenth aspect, the contact surface area between he metallic pin and the reflecting film is ensured, and in the electrooptic element, the fluctuation in the electric field that appears as a fluctuation in the state of polarization can be favorably detected.